Revised historical Northern Hemisphere black carbon emissions based on inverse modeling of ice core records

Black carbon emitted by incomplete combustion of fossil fuels and biomass has a net warming effect in the atmosphere and reduces the albedo when deposited on ice and snow; accurate knowledge of past emissions is essential to quantify and model associated global climate forcing. Although bottom-up inventories provide historical Black Carbon emission estimates that are widely used in Earth System Models, they are poorly constrained by observations prior to the late 20th century. Here we use an objective inversion technique based on detailed atmospheric transport and deposition modeling to reconstruct 1850 to 2000 emissions from thirteen Northern Hemisphere ice-core records. We find substantial discrepancies between reconstructed Black Carbon emissions and existing bottom-up inventories which do not fully capture the complex spatial-temporal emission patterns. Our findings imply changes to existing historical Black Carbon radiative forcing estimates are necessary, with potential implications for observation-constrained climate sensitivity.


Precipitation in CERA
: Names, locations, heights, corresponding CERA topographical height and CERA annual mean precipitation, measurement method and available references for the 13 ice cores used in this study.

Emissions
For transport into the Arctic, high-and mid-latitude emissions are more relevant than global emissions. Therefore,

Variation of BC emissions related to measured and modeled BC deposition in ice cores
For comparing the magnitude of modeled and observed BC deposition fluxes, we focus on the last 10 years (1990-1999) of our record when emission uncertainties are expected to be smallest.  Figure S4 shows BC emission contributions based on CMIP6 (the newer one of the two inventories) emission 5 data and FLEXPART emission sensitivities combined for wet and dry deposition, for two ice cores with pronouncedly different characteristics: D4 (Fig S4a, b), which has its source region over both North America and western Europe (compare with Fig. 1b) and Akademii Nauk (Fig. S4c, d), which receives emissions mostly from wide parts of Eurasia but has little sensitivity to North American emissions (see Fig. 1c). As both emission magnitude and distribution changed over the 150 years investigated, we plot the emission contributions separately for 10 two ten-year periods (1910-1920 and 1980-1990, respectively). At the beginning of the 20th century (Fig S4a), there are two source contribution hot spots for D4, one over the North American East coast and one over western 6 Europe (primarily Great Britain). At the end of the 20th century (Fig. S4b), the source contributions from both these regions are reduced substantially, but additional contributions come from emissions in Eastern Europe and Northern Siberia (gas flaring), where the emission sensitivity is quite low. This eastward shift of emission contributions over Eurasia is even more pronounced for Akademii Nauk. There, source contributions at the beginning of the 20th century are mainly from western Europe (Fig. S4c), but at the end of the 20th century (Fig. S4d), In order to see the emission contributions from different latitudes on the deposition at the ice core locations, we sum up the contributions for all sites over 10-degree latitude bands in Fig S4 (panels a-d)

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A time series of simulated BC deposition, split by emission region origin, is created by integrating the source contribution maps for CMIP6 emissions, shown exemplarily for two ice core sites in Fig. S4, over defined emission regions (Fig. S5). We distinguish between North American, European, Russian and Southern Asian and the rest of the world emissions (Fig. S5, last panel). The high-altitude Greenland ice cores receive about equal contributions from Europe and North America. Averaged over the 100 years period, North America contributes be-

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We find that wet deposition dominates at most sites (Fig. S6, upper panel). This is particularly true in winter, when dry deposition is almost completely negligible. The inefficiency of dry deposition in winter is related to the high static stability in the Arctic lower troposphere, which almost completely stops the dry deposition process for small BC particles, which have no significant gravitational settling. Thus, dry deposition in winter contributes less than 1% of total winter deposition. At the same time, winter and spring wet deposition make the largest contribu-

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tions to the total annual deposition at all Arctic sites (Fig. S6). While summer deposition is generally lower, dry and wet deposition are comparable at all sites during this season.

tion (upper panel), and displayed separately for anthropogenic and biomass burning (BB) contributions (lower panel).
To evaluate the evolution and magnitude of the BC emissions, we plot both the CMIP5 and CMIP6 (1850-1999) emissions, split into different latitude bands, against the ice core observations (Fig. S7 and Fig. S8, respectively).
As most ice cores are located in the Arctic, emissions from the higher latitude bands are more important and therefore we stack the emissions starting from the highest to the lowest latitudes with darkest shading for the 5 highest latitude bands. Most of the emission contributions to Arctic ice core deposition are from latitudes north of 30°N (Fig. S1e, f), and therefore we further emphasize the emissions north of 30°N with a white line. decrease. Such high late-20 th century BC deposition fluxes were also observed in lake sediments from northwestern arctic Russia, and were attributed to high emissions in the Russian Arctic flaring region (Ruppel et al., 2021), an important source region for both Akademii Nauk and Holtedahlfonna (Fig. 2).
The timing of the observed early 20 th century BC deposition maximum in Greenland ice cores (ACT2, D4, Sum-20 mit, Tunu, NEEM) is much better captured by the model when using CMIP5 emissions (Fig. S7) than CMIP6 emissions (Fig. S8). These sites are most sensitive to emissions in North America ( Fig. 1; S5), and indeed in North America the highest CMIP5 emissions of the whole period occurred already in 1910. Our comparison confirms that there must have been a strong North American BC emission peak already in the early 20th century declining from there onwards. In contrast, the CMIP6 FLEXPART simulations do not capture the observed early 20 th century deposition maximum, suggesting that the temporal evolution of BC emissions in North America in the 20th 5 century is not correctly represented in the CMIP6 emission inventory.
The measured and the CMIP6 modeled deposition fluxes at NEEM are similar during the first 50 years, but thereafter the modeled deposition values are higher than the observed ones (Fig. S8). This is not the case at Tunu and Summit, which exhibit much lower measured fluxes and BC deposition at both sites is significantly overestimated by the model throughout the entire record.

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In Holtedahlfonna shows a distinct peak around 1910 and an even stronger peak at the end of the 20th century. These differences can be partly attributed to differences in analytical methods used (SP2 vs. thermo-optical), but may also reflect some local BC source-receptor differences, as well as differences in the state of preservation of the BC signals at the two sites (due to site-specific factors) and/or spatial noise in atmospheric BC deposition, which is 30 presently not well-quantified in Svalbard.
This example illustrates how valuable multiple ice core records are for a comprehensive spatial view, as in some cases local meteorological conditions may affect what particles (free troposphere vs. boundary layer) are recorded in the respective ice core.